The primary objective of this study was to investigate structural changes, as measured by joint space narrowing (JSN), within the knee joint during treatment with intra-articular sodium hyaluronate (HA) of molecular weight 500-730 kDa in patients with osteoarthritis (OA) of the knee. Patients received a weekly intra-articular injection of either 20 mg2/ml HA or a 2 ml vehicle placebo (saline) for three weeks. This course was repeated twice more at four-monthly intervals. Concomitant treatment with analgesics or NSAIDs was allowed. The primary efficacy measure was the reduction in mean joint space width (JSW) of the medial compartment at 52 weeks. A total of 408 patients were randomised and 319 completed the one-year study (HA: n=160, placebo: n=159); 273 of the 319 were included in the primary analysis. Analysis of variance on these 273 patients did not show a statistically significant difference between the two treatment groups. However, there was a significant difference in response to treatment in terms of the baseline JSW (p=0.01), indicating that outcome of treatment may depend on-baseline JSW. Therefore, a subgroup analysis by baseline JSW was conducted. This compared patients with a JSW >4.6 mm with those with a JSW <4.6 mm. In those with radiologically milder disease at baseline and receiving HA, the JSN was significantly reduced compared with placebo (p=0.02). In patients with radiologically more severe disease there was no difference in JSN between the two treatments. Although, in this one-year study, no overall treatment effect was seen, those with radiologically milder disease at baseline had less progression of joint space narrowing when treated with HA.